This invention relates to fluid control valves, and more particularly to gate valves for use where elevated temperatures and pressures are encountered.
Bi-directional sealing gate valves of various designs are commonly used for controlling the flow of fluids in land based and subsea oil or gas wells and in pipelines of the petroleum and petrochemical industries. When two such valves are installed in series and fluid is trapped between the two closed gates, a line pressure buildup can occur due to fluid expansion that results if the temperature of the fluid increases. This increase in temperature typically occurs when, for example, valves at a surface (land or platform) wellhead are closed at nighttime and the ambient temperature rises the following day. Pressure buildup can be substantial with temperature increases of only 50-75 degrees Farenheit, but is much greater in the event of a fire which could raise the fluid temperature to 1000-1200 degrees Farenheit or more. In either event, the pressure could build up to an unsafe level and cause the valves to leak or even rupture. Fluid-trapping and the resulting dangers that are created when the ambient temperature rises significantly also exist in a single valve installation if the valve design does not include a means for self-relieving the body cavity pressure.
Attempts to provide solutions to the foregoing problems include the provision for uni-directional sealing of one valve in the two valve series so that pressure build-up can always by-pass back through the uni-directional valve. This solution, however, has the disadvantage of preventing pressure testing against the uni-directional valve in a reverse direction, an important and desirable feature for most oil-field applications.